1. Field of the Invention
The present invention relates to an eyepiece optical system for a single lens reflex camera.
2. Description of the Related Art
In a single lens reflex camera, an erecting optical system using a hollow penta mirror instead of a penta prism is well known. Since a penta mirror has the reflection-optical-path in the air, the equivalent air thickness along the optical axis cannot be made shorter up to 1/n times (n: the refractive index of a penta prism to be compared) of the equivalent air thickness of the penta prism. Conversely, since the reflection-optical-path is `n` times as long as that of the penta prism having the same geometrical distance of the reflection-optical-path, the focal length of an eyepiece optical system is made longer, and a finder magnification is lowered. Accordingly, a special design is required for the eyepiece optical system. For example, Japanese Unexamined Patent Publication Nos. 9-146019, and United States Patent Publication No.5,136,427 propose a two-lens-element eyepiece optical system constituted by a positive lens element and a negative lens element, in this order from the side of a penta mirror.
However, the eyepiece optical system in Japanese Unexamined Patent Publication No. 9-146019 is for a single lens reflex camera which uses a film type smaller than a 135-type film, more concretely, for an APS (advanced photo system) camera. Therefore the eyepiece optical system cannot be converted to an eyepiece optical system for a single lens reflex camera in which the a 135-type film is used. Furthermore, the eyepiece optical systems in U.S. Pat. No. U.S. Pat. No. 5,136,427 is not provided with an optical system having a diopter adjusting function. Of course it is theoretically possible to change the diopter by moving at least a portion of lens elements constituting an eyepiece optical system. However, since an optical system having a diopter adjusting function is not originally considered, if an attempt is made to satisfy a condition for maintaining a sufficient eye relief (the distance from the final surface, at the side of a photographer's eye, of the eyepiece optical system to the eyepoint) with respect to the light rays from the periphery of the field-of-view, a range of adjustment is smaller even if the lens diameter is increased, whereby an effect on a diopter adjustment is insufficient.
Furthermore, U.S. Pat. No. 5,313,327 proposes an eyepiece optical system for a penta mirror, the eyepiece optical system of which includes a positive single lens element. Also, this eyepiece optical system can theoretically change the diopter by moving the positive single lens element. However, since the amount of change in the diopter is small with respect to the amount of movement of the positive single lens element, a sufficient range of diopter adjustment cannot be obtained, and even in such a range, a sufficient eye relief with respect to the light rays from the periphery of the field-of-view cannot be maintained.
Moreover, in any of the above mentioned eyepiece optical systems, the change in the apparent visual angle upon the adjustment of the diopter has not been considered. The adjusting of the diopter is usually performed while a photographer is looking through the finder, and accordingly, is unpleasant for the photographer to observe the apparent visual angle being varied. In particular, when the diopter is suitably set for the photographer, if the apparent visual angle is narrowed, the feel of unpleasantness may be intensified.